1. Field of the Invention
The present invention relates to a method for reducing heavy metals, in particular mercury, present in flue gases, comprising a stage of bringing the flue gases into contact with a sorbent solid material in the dry state.
2. Description of the Prior Art
The term “heavy metals” refers to the following chemical compounds listed here for illustrative purposes only: francium, radium, lanthanides, actinides, zirconium, hafnium, rutherfordium, vanadium, niobium, tantalum, dubnium, chromium, molybdenum, tungsten, manganese, technetium, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, mercury, gallium, indium, thallium, germanium, tin, lead, arsenic, antimony, bismuth, polonium. The reduction method according to the invention mainly concerns the most common heavy metals, namely lead, chromium, copper, Mn, antimony, arsenic, cobalt, nickel, vanadium, cadmium, thallium and mercury, preferably lead, thallium, cadmium, mercury, and in particular mercury.
In the prior art the reduction in heavy metals, in particular mercury, present in flue gases is generally achieved by means of carbonated compounds such as active carbon or lignite coke. They may be used either as such or in a mixture with an alkaline absorbent in a fixed bed in granular form or by injection into the gas in powder form; the solid particles are then trapped upstream for example in the textile filter where the action is prolonged.
The efficacy of carbonated compounds for capturing these metals is unanimously acknowledged. Nonetheless the use of these carbonated compounds in flue gases has two major drawbacks:                an increase in the carbon content in the dust from filtration of these gases, the carbon content being strictly regulated;        the risk of flammability, which increases the higher the temperature of the gas to be cleaned.        
An improvement made by the person skilled in the art to solve the problems of combustion of carbonated compounds has been to use them in a mixture with flammability retarders such as lime. Unfortunately this improvement, although effectively reducing the problem of combustion of carbonated compounds, has not solved it completely. In fact hot points can even appear at low temperatures (for example 150° C.), in particular in the presence of infiltration air in zones where carbonated compounds are accumulated. The carbonated compounds are also expensive compounds and the stage implementing such carbonated compounds is difficult to integrate into a complete process of flue gas treatment. In fact today a complete process, due to ever stricter new standards, must often also eliminate nitrogenated products from the flue gases. The elimination of nitrous oxides by the catalytic route is generally performed at a higher gas temperature (over 200° C.). For safe compatibility with a process stage using carbonated compounds, cooling of the flue gases and their heating must often be alternated. This represents a loss of economic benefit and time, and a significant energy loss. It is hence difficult to integrate carbonated compounds in a gas treatment process, given the combustion problems posed by these compounds.
Documents “ES 8704428” and “GIL M. ISABEL; ECHEVERRIA, SAGRARIO MENDIOROZ; MARTIN-LAZARO, PEDRO JUAN BERMEJO; ANDRES, VICENTA MUNOZ, Mercury removal from gaseous streams. Effects of adsorbent geometry, Revista de la Real Academia de Ciencias Exactas, Fisicas y Naturales (Espana) (1996), 90(3), pp 197-204” reveal that it is possible to avoid carbon in reducing heavy metals, in particular mercury, by using sulphur as a reagent. The sulphur is deposited on a mineral support such as natural silicates. Such formations thus remedy the said drawbacks of carbonated phases. In this case the silicate is considered as an inert support in relation to the pollutant to be captured; the latter is trapped by reaction with the sulphurous compound, generally to form a sulphide.
Unfortunately the silicates functionalized by sulphurous compounds are often dangerous, heavy and costly to produce, which penalizes their use. For example document ES 8704428 discloses sulphurisation of a silicate by oxidation reaction of hydrogen sulphide (H2S) in a clearly defined molar proportion, with a view to adsorbing the elementary sulphur on the said silicate. The handling of toxic hydrogen sulphide (H2S) is a dangerous operation and the strict molar proportion necessary to avoid any subsequent oxidation reaction is extremely restrictive.